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Leaves in the Wind:
Steven R. CranmerUniversity of Colorado Boulder, LASP
A. A. van Ballegooijen, L. N. Woolsey, A. Schiff, S. Van Kooten, C. Gilbert,
S. E. Gibson, C. E. DeForest, J. L. Kohl, S. Saar, M. P. Miralles, S. P. Owocki
The Variety of Radiative & MHD fluctuationsin Rotating Solar/Stellar Outflows
Leaves in the Wind:
Steven R. CranmerUniversity of Colorado Boulder, LASP
Outline:
1. The Sun: convection → coronal heating?
2. Cool stars: generalizing the solar case; accretion
3. Massive stars: radiation pressure & pulsations
A. A. van Ballegooijen, L. N. Woolsey, A. Schiff, S. Van Kooten, C. Gilbert,
S. E. Gibson, C. E. DeForest, J. L. Kohl, S. Saar, M. P. Miralles, S. P. Owocki
The Variety of Radiative & MHD fluctuationsin Rotating Solar/Stellar Outflows
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Why do we care?
• Consequently, they affect the formation & habitability of planets, too.
• Stellar winds affect how stars &
galaxies evolve… from pre-main-
sequence accretion to post-main-
sequence “death” & mass recycling.
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Why do we care?
• Consequently, they affect the formation & habitability of planets, too.
• In our own solar system, “space weather”
affects satellites, power grids, pipelines,
and safety of astronauts & high-altitude
airline crews.
• If you can understand how plasmas behave
in turbulent, expanding stellar
atmospheres, you’ll have a superb
grounding in many fields.
• Stellar winds affect how stars &
galaxies evolve… from pre-main-
sequence accretion to post-main-
sequence “death” & mass recycling.
Stellar winds across the H-R Diagram
Stellar winds across the H-R Diagram
Massive stars: radiation-driven
winds
Stellar winds across the H-R Diagram
Massive stars: radiation-driven
winds
Cool luminous stars:
pulsation/dust-driven winds?
Stellar winds across the H-R Diagram
Massive stars: radiation-driven
winds
Solar-type stars: coronal winds
(driven by MHD turbulence?)
Cool luminous stars:
pulsation/dust-driven winds?
1. The Sun: convection → coronal heating?
2. Cool stars: generalizing the solar case; accretion
3. Massive stars: radiation pressure & pulsations
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Convection produces granulation
• Unstable convective
overturning drives p-mode
internal pulsation modes:
largely evanescent at surface.
• The uppermost convection cells are visible as “granules,” and strong-field
magnetic flux tubes are jostled (mostly) horizontally…
Spruit
(1984)
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Flux tubes (eventually) fill the corona
Analyzing some individual
thin-tube oscillations has
led to novel ways to
measure the magnetic field
(“coronal seismology”).
Nakariakov &
Verwichte (2004)
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Flux tubes (eventually) fill the corona
Analyzing some individual
thin-tube oscillations has
led to novel ways to
measure the magnetic field
(“coronal seismology”).
Nakariakov &
Verwichte (2004)
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
MHD waves expand out into the corona
With good instrumentation, imaging
& spectroscopy can resolve plasma
fluctuations in multiple ways…
• Transverse Alfvén waves dominate, with periods of order 3-5 minutes.
• Intensity modulations . . .
• Motion tracking in images . . .
• Doppler shifts . . .
• Doppler broadening . . .
• Radio sounding . . .
Tomczyk
et al.
(2007)
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Measured Alfvénic fluctuations
• Cranmer & van Ballegooijen (2005) collected a range of observational data…
Hinode/SOT
G-band
bright
points
SUMER & EIS
Helios &
Ulysses
UVCS/SOHO
Undamped (WKB) waves
Damped (non-WKB) waves
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Measured Alfvénic fluctuations
• Cranmer & van Ballegooijen (2005) collected a range of observational data…
Hinode/SOT
G-band
bright
points
SUMER & EIS
Helios &
Ulysses
UVCS/SOHO
Undamped (WKB) waves
Damped (non-WKB) waves
EIS
(Hahn et al.
2013)
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Can turbulence explain coronal heating?
Convection shakes & braids magnetic
field lines in a diffusive “random walk”
Alfvén waves propagate up...
partially reflect
back down...
...and they undergo an MHD
turbulent cascade, from large
to small eddies, eventually
dissipating in intermittent
stochastic “nanoflares”
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Can turbulence explain coronal heating?
• If the cascade is driven & time-steady, the rate of stirring should = rate of cascade = rate of dissipation & heating.
• MHD simulations inspire phenomenological scalings for the stirring/cascade rate:
(e.g., Iroshnikov 1963; Kraichnan 1965; Strauss 1976;
Shebalin et al. 1983; Hossain et al. 1995;
Goldreich & Sridhar 1995; Matthaeus et al. 1999;
Dmitruk et al. 2002; Chandran 2008)
wavenumber
Po
wer
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Can turbulence explain coronal heating?
• If the cascade is driven & time-steady, the rate of stirring should = rate of cascade = rate of dissipation & heating.
• MHD simulations inspire phenomenological scalings for the stirring/cascade rate:
(e.g., Iroshnikov 1963; Kraichnan 1965; Strauss 1976;
Shebalin et al. 1983; Hossain et al. 1995;
Goldreich & Sridhar 1995; Matthaeus et al. 1999;
Dmitruk et al. 2002; Chandran 2008)
wavenumber
Po
wer
• When plugged into self-consistent
solutions for coronal heating & solar
wind acceleration, it seems to work!
(Cranmer et al. 2007).
• Including rotation produces realistic
3D structure (Cranmer et al. 2013).
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Kinetic consequences…
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Kinetic consequences…
• When eddies reach gyroradius scales, does
the anisotropic cascade prefer:
k
k
Observing collisionless heating rates high up (e.g., UVCS/SOHO) reveals indirect information about how wave dissipation heats particles . . .
ion cyclotron waves?
kinetic Alfvén waves?
magnetosonic whistlers?
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Kinetic consequences…
• When eddies reach gyroradius scales, does
the anisotropic cascade prefer:
k
k
Observing collisionless heating rates high up (e.g., UVCS/SOHO) reveals indirect information about how wave dissipation heats particles . . .
Does the “wave” picture break down
altogether when the turbulence is organized
into coherent current sheets?
ion cyclotron waves?
kinetic Alfvén waves?
magnetosonic whistlers?
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
MHD waves generated “higher up?”
Not all coronal & solar wind fluctuations come directly from the solar surface…
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
MHD waves generated “higher up?”
Not all coronal & solar wind fluctuations come directly from the solar surface…
• The coronal magnetic field evolves via
magnetic reconnection between ever-
changing magnetic flux systems.
• Some forms of reconnection can launch
MHD waves (Lynch et al. 2014; Moore
et al. 2015).
• Strong shears between fast & slow solar
wind (and CMEs!) can be unstable to
wave growth via Kelvin-Helmholtz
instabilities (Foullon et al. 2011; Ofman
& Thompson 2011).
1. The Sun: convection → coronal heating?
2. Cool stars: generalizing the solar case; accretion
3. Massive stars: radiation pressure & pulsations
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Applying turbulence theory to solar-type stars
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Applying turbulence theory to solar-type stars
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Evolution of inflows & outflows
0.01 0.1 1 2 3 4 10
AGE (Billions of years)
Mas
s gai
ned
or
lost
(M
oons
/yr)
100
10
1
0.1
0.01
0.001
0.0001
0.00001
0.000001
Accretion rate
T Tauri phase
Present-day Sun
Red giant
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Young stars: 2 sources of turbulence
• T Tauri protostars are convectively unstable… they generate their own waves.
• But the accretion is variable! Clumps of plasma impact the star… and induce “externally driven” surface turbulence.
• Cranmer (2008, 2009) modeled the resulting winds & X-ray emission.
Impact-generated “ripples” are similar to EUV waves observed on the Sun after strong flares…
1. The Sun: convection → coronal heating?
2. Cool stars: generalizing the solar case; accretion
3. Massive stars: radiation pressure & pulsations
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Massive star winds: radiative driving
• Castor, Abbott, & Klein (1975) worked out how a
hot star’s radiation can accelerate a time-steady
wind, even if gravity >> continuum radiation force.
• Spectral lines are the key!
• Bound electron resonances have higher cross-
sections than free electrons (i.e., spectral lines
dominate the opacity κν)
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
Massive star winds: radiative driving
• Castor, Abbott, & Klein (1975) worked out how a
hot star’s radiation can accelerate a time-steady
wind, even if gravity >> continuum radiation force.
• Spectral lines are the key!
• Bound electron resonances have higher cross-
sections than free electrons (i.e., spectral lines
dominate the opacity κν)
• In the accelerating wind, narrow opacity sources
become Doppler shifted with respect to star’s
photospheric spectrum.
• Acceleration thus depends on velocity & velocity
gradient! This turns “F=ma” on its head!
(Nonlinear feedback...) ν
κν
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
New forces → new wave modes
• Radiative acceleration is proportional to (dv/dr) and wind density…
usually neglected;
important for low freq’s
Steeper gradients → stronger line forces.
The Abbott (1980) speed UA can be supersonic
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
New forces → new wave modes
• Radiative acceleration is proportional to (dv/dr) and wind density…
usually neglected;
important for low freq’s
Steeper gradients → stronger line forces.
The Abbott (1980) speed UA can be supersonic
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
New forces → new wave modes
• Massive stars undergo low-frequency pulsations. Do waves “leak out?”
• Proper treatment requires a non-WKB model of Abbott waves (Cranmer 2007).
Strongly nonlinear pulsations
saturate; radiation forces
produce “kinks” in v(r)
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
New forces → new wave modes
• For a rotating star with “spots,” the
nonlinear kinks produce corotating
interaction regions (CIRs).
• CIRs appear to be visible in spectral
lines formed in rotating winds
(Cranmer & Owocki 1996).
Radiative & MHD fluctuations in Stellar Winds S. R. Cranmer, GTP Workshop, Aug. 17, 2016
New forces → new wave modes
• A more accurate treatment of the
radiation force shows that small
wavelengths are strongly unstable to
rapid growth (Owocki & Rybicki
1984, 1985, 1986, …)
• The resulting shocks appear to explain
X-rays seen around most O stars.
• For a rotating star with “spots,” the
nonlinear kinks produce corotating
interaction regions (CIRs).
• CIRs appear to be visible in spectral
lines formed in rotating winds
(Cranmer & Owocki 1996).
Conclusions
• Waves are excellent diagnostics of stellar outflows.
@solarstellar
• Within an order of magnitude, theories aren’t
doing too badly in predicting observed
properties of solar & stellar winds.
• However, there’s still much to do . . .
• Understanding is greatly aided by ongoing
collaboration between the solar physics,
plasma physics, and astrophysics communities.